Not all compilerplugins/clang/*.cxx depend on config_clang.h (e.g., check.cxx
doesn't), so this mechanism trying to rebuild compilerplugins once the
underlying Clang installation changes doesn't work reliably in practice (just
debugged through this with Miklos on IRC, and it wasn't the first time that
`make distclean` fixed compilerplugins for somebody after they upgraded their
Clang installation).  Removing the brittle mechanism shows that plugin.hxx
doesn't actually depend on config_clang.h.

(There's a second mechanism trying to rebuild compilerplugins once the
underlying Clang installation changes, namely

> # Clang most probably doesn't maintain binary compatibility, so rebuild when clang changes.
> $(CLANGOUTDIR)/clang-timestamp: $(CLANGDIR)/bin/clang$(CLANG_EXE_EXT)
>         $(QUIET)touch $@

in compilerplugins/Makefile-clang.mk, but that doesn't work reliably either, as
it depends on the newly installed clang executable being newer than our
clang-timestamp file, which will be the case for self-built Clang installations,
but not necessarily when updating e.g. a distro-provided Clang installation.)

Change-Id: Ie576f14356b3f0e55444375095c86aa851404bf3
Reviewed-on: https://gerrit.libreoffice.org/64623
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>