Don't disable ccache when building with coverage support

This was working around a bug in ccache that was fixed in ccache
3.2.2 (released 2015-05-10).  (Users of older ccache versions can
continue to set CCACHE_DISABLE themselves.)

Discussion: https://www.postgresql.org/message-id/20190530191130.GA24528@alvherre.pgsql

src/Makefile.global.in

@@ -481,11 +481,6 @@ GCOV = @GCOV@
 LCOV = @LCOV@
 GENHTML = @GENHTML@
 
-ifeq ($(enable_coverage),yes)
-# ccache loses .gcno files
-export CCACHE_DISABLE = 1
-endif
-
 # Feature settings
 
 DEF_PGPORT = @default_port@