Makefile: avoid overzealous removal of compile_commands.json (#1157)

Our logic to generate a compilation database is copied (almost
verbatim) from Git's Makefile,

However, one difference is that before creating compile_commands.json
we always remove it.
This doesn't seem very useful, plus it thwarts the logic two lines
below to overwrite compile_commands.json only if we successfully
created a non-empty file.  I believe I added this while debugging
this logic.

I'm not sure in which scenarios this actually makes a difference,
but it seems wrong to keep it.

Makefile

@@ -126,7 +126,6 @@ doc-html: $(HTMLDOC)
 ifeq ($(GENERATE_COMPILATION_DATABASE),yes)
 all: compile_commands.json
 compile_commands.json:
-	@rm -f $@
 	$(QUIET_GEN)sed -e '1s/^/[/' -e '$$s/,$$/]/' $(compdb_dir)/*.o.json > $@+
 	@if test -s $@+; then mv $@+ $@; else $(RM) $@+; fi
 endif