Remove unmaintained Dockerfile syntax for Kate

We don't support this and no maintainers uses Kate.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

contrib/syntax/kate/Dockerfile.xml

@@ -1,67 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE language SYSTEM "language.dtd">
-<!-- Dockerfile syntaxfile v1.0 by James Turnbull <james@lovedthanlost.net> -->
-<language name="Dockerfile" section="Other"
-          version="1.0" kateversion="2.4"
-          extensions="Dockerfile"
-          mimetype="text/plain"
-          author="James Turnbull (james@lovedthanlost.net)"
-          license="GPL">
-  <highlighting>
-    <list name="keywords">
-      <item> ADD </item>
-      <item> ARG </item>
-      <item> CMD </item>
-      <item> COPY </item>
-      <item> ENTRYPOINT </item>
-      <item> ENV </item>
-      <item> EXPOSE </item>
-      <item> FROM </item>
-      <item> HEALTHCHECK </item>
-      <item> LABEL </item>
-      <item> MAINTAINER </item>
-      <item> ONBUILD </item>
-      <item> RUN </item>
-      <item> SHELL </item>
-      <item> STOPSIGNAL </item>
-      <item> USER </item>
-      <item> VOLUME </item>
-      <item> WORKDIR </item>
-    </list>
-    <contexts>
-      <context name="normal" attribute="Normal" lineEndContext="#stay">
-        <DetectSpaces/>
-        <DetectChar attribute="Comment" context="Comment" char="#"/>
-        <keyword attribute="Keyword"  context="#stay" String="keywords"/>
-        <DetectIdentifier/>
-        <DetectChar attribute="String" context="string&quot;" char="&quot;"/>
-        <DetectChar attribute="String" context="string'" char="'"/>
-      </context>
-      <context attribute="Comment" lineEndContext="#pop" name="Comment">
-        <LineContinue attribute="Comment" context="#stay" />
-      </context>
-      <context name="string&quot;" attribute="String" lineEndContext="#pop">
-        <LineContinue attribute="Operator" context="#stay"/>
-        <DetectChar attribute="String" context="#pop" char="&quot;"/>
-        <DetectChar attribute="Operator" context="dollar" char="$"/>
-      </context>
-      <context name="string'" attribute="String" lineEndContext="#pop">
-        <LineContinue attribute="String" context="#stay"/>
-        <DetectChar attribute="String" context="#pop" char="'"/>
-        <DetectChar attribute="Operator" context="dollar" char="$"/>
-      </context>
-    </contexts>
-    <itemDatas>
-      <itemData name="Normal"    defStyleNum="dsNormal" spellChecking="0"/>
-      <itemData name="Keyword"   defStyleNum="dsKeyword" spellChecking="0"/>
-      <itemData name="Comment"   defStyleNum="dsComment"/>
-      <itemData name="String"    defStyleNum="dsString" spellChecking="0"/>
-    </itemDatas>
-  </highlighting>
-  <general>
-    <comments>
-      <comment name = "singleLine" start = "#"/>
-    </comments>
-  </general>
-</language>
-<!-- kate: space-indent on; indent-width 2; replace-tabs on; -->