Consolidate custom clean targets for coverage files

Consolidate custom clean targets for coverage files

As reported by Mark:

Since b2f225d6b ("Consolidate and add files to clean target variables")
autoreconf (automake) produces these warnings:

debuginfod/Makefile.am:130: warning: CLEANFILES multiply defined in condition TRUE ...
config/eu.am:138: ... 'CLEANFILES' previously defined here
debuginfod/Makefile.am:32:   'config/eu.am' included from here
libasm/Makefile.am:91: warning: CLEANFILES multiply defined in condition TRUE ...
config/eu.am:138: ... 'CLEANFILES' previously defined here
libasm/Makefile.am:30:   'config/eu.am' included from here
libcpu/Makefile.am:105: warning: CLEANFILES multiply defined in condition TRUE ...
config/eu.am:138: ... 'CLEANFILES' previously defined here
libcpu/Makefile.am:30:   'config/eu.am' included from here
libdw/Makefile.am:156: warning: CLEANFILES multiply defined in condition TRUE ...
config/eu.am:138: ... 'CLEANFILES' previously defined here
libdw/Makefile.am:30:   'config/eu.am' included from here
libelf/Makefile.am:142: warning: CLEANFILES multiply defined in condition TRUE ...
config/eu.am:138: ... 'CLEANFILES' previously defined here
libelf/Makefile.am:30:   'config/eu.am' included from here
src/Makefile.am:47: warning: CLEANFILES multiply defined in condition TRUE ...
config/eu.am:138: ... 'CLEANFILES' previously defined here
src/Makefile.am:19:   'config/eu.am' included from here
tests/Makefile.am:891: warning: CLEANFILES multiply defined in condition TRUE ...
config/eu.am:138: ... 'CLEANFILES' previously defined here
tests/Makefile.am:19:   'config/eu.am' included from here

This is because config/eu.am defines a default "CLEANFILES".

However, the list is only for the removal of coverage files.
Since there is an exising custom clean rule for coverage in Makefile.am
because there is a directory involved as well as files,
further simplification of these rule definitions can resolve this.

Define cleaning targets for coverage files
in a single place and as a common definition
for all generated Makefiles using a custom rule.
This allows for avoiding the use of "CLEANFILES"
variables for anything related to removing coverage files,
and removing the usage of this variable where there is a conflict,
as it is no longer needed to be defined there.

    * Makefile.am: remove variables and custom clean target.
    * config/eu.am: remove clean variables, add variables
      and custom clean target based on removed definitions.

Fixes: b2f225d6b ("Consolidate and add files to clean target variables")
Reported-by: Mark Wielaard <mark@klomp.org>
Signed-off-by: Michael Pratt <mcpratt@pm.me>