Remove no-longer functional --with-alloc

...after db354dfa "the custom SAL allocator is
no longer used"

Change-Id: Ia897e2d3cf20bb240dcc3eed801b81f86267dc6a
Reviewed-on: https://gerrit.libreoffice.org/58475
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

configure.ac

@@ -2189,11 +2189,6 @@ AC_ARG_WITH(build-version,
 with_build_version=$withval,
 )
 
-AC_ARG_WITH(alloc,
-    AS_HELP_STRING([--with-alloc],
-        [Define which allocator to build with (choices are internal, system).]),
-,)
-
 AC_ARG_WITH(parallelism,
     AS_HELP_STRING([--with-parallelism],
         [Number of jobs to run simultaneously during build. Parallel builds can