unit-tester: Drop the old unit-tester libcharon plugin

While it has some tests that we don't directly cover with the new unit tests,
most of them require special infrastructure and therefore have not been used
for a long time.

src/libcharon/Makefile.am

@@ -600,13 +600,6 @@ if MONOLITHIC
 endif
 endif
 
-if USE_UNIT_TESTS
-  SUBDIRS += plugins/unit_tester
-if MONOLITHIC
-  libcharon_la_LIBADD += plugins/unit_tester/libstrongswan-unit-tester.la
-endif
-endif
-
 if USE_XAUTH_GENERIC
   SUBDIRS += plugins/xauth_generic
 if MONOLITHIC