configure.in: duplicated linker flags

* configure.in (with_jemalloc): get rid of duplicated linker flags,
  as AC_CHECK_LIB adds it.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47127 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.in

@@ -1184,7 +1184,6 @@ AS_IF([test "x$with_jemalloc" = xyes],[
   AS_IF([test "x$with_jemalloc" = xyes],
     [
       ac_cv_func_malloc_usable_size=yes
-      LIBS="-ljemalloc $LIBS"
     ],
     [AC_MSG_ERROR([jemalloc requested but not found])
   ])