Bug 1426327 - Clean up PATH for mingw builds. r=gps

The PATH defined for mingw builds was cargo culted, lacks
/usr/local/bin, and contains things that are pretty much useless these
days, now that we're off buildbot. Similarly, LD_LIBRARY_PATH is
useless.

While many other similar changes could be done to the other mozharness
configurations, figuring out which ones are used under what
circumstances is more work than I'm ready to put (I started, but I
stopped when I encountered jobs that don't even run on try or central).

--HG--
extra : rebase_source : dbcbcf9ba80ebb2858c3d47a186daa367afa2988

testing/mozharness/configs/builds/releng_base_windows_32_mingw_builds.py

@@ -59,10 +59,7 @@ config = {
         'MOZ_CRASHREPORTER_NO_REPORT': '1',
         'LC_ALL': 'C',
         # 32 bit specific
-        'PATH': '/tools/buildbot/bin:/bin:/usr/bin:/usr/local/sbin:\
-/usr/sbin:/sbin:/tools/git/bin:/tools/python27/bin:\
-/tools/python27-mercurial/bin:/home/cltbld/bin',
-        'LD_LIBRARY_PATH': "/tools/gcc-4.3.3/installed/lib",
+        'PATH': '/usr/local/bin:/bin:/usr/bin',
     },
     'upload_env': {
         # stage_server is dictated from build_pool_specifics.py