unix-Makefile.tmpl: omit lib<rary>.a updates from directory targets.

Since corresponding rule was removed from windows-makefile.tmpl out
of necessity, question popped if it's appropriate to harmonize even
unix-Makefile.tmpl. Note that as long as you work on single directory
'make lib<rary>.a' is effectively equivalent to 'make <dir/ectory>'
prior this modification.

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