profiles/arch/*/make.defaults: tweak RUBY_TARGETS override

profiles/arch/*/make.defaults: tweak RUBY_TARGETS override

Unfortunately RUBY_TARGETS does not have merge semantics
and just overrides value from parent profile. Thus
    RUBY_TARGETS="ruby23 -ruby24"
is not valid and yields error spam like:
    Invalid '-' operator in non-incremental variable 'RUBY_TARGETS': '-ruby24'

Just drop RUBY_TARGETS="-ruby" part. The change should be a no-op.

Bug: https://bugs.gentoo.org/671064
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>