Remove unused, undocumented clean-shared target

Also removes the make variable SHARED_LIBS_LINK_EXTS, only used by
the clean-shared target.

When shared library linking was moved to the separate Makefile.shared
in commit 30afcc07

, this target was
skipped.  Prior to that commit, clean-shared was invoked as a
dependency of build-shared, but afterward it was no longer referenced
anywhere in the tree.

Instead of porting the functionality over to Makefile.shared, just
remove it entirely, as it appears to be unused.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>