Remove "CC" from CWRAPPERS_ALIASES.cxx.

Motivation: with "USE_CWRAPPERS=yes" on the default case-insensitive
OS X filesystem, linking the wrappers was failing, stopping the
build. For example:

    ===> Creating toolchain wrappers for mess822-0.58nb3
    ln: .../pkgsrc-current/mail/mess822/work/.cwrapper/bin/cc: File exists
    *** Error code 1

    Stop.

I never noticed this with the old wrappers, but they've also been
failing to link "CC" as an alias to "c++"; it's just that the failure
has been silent and I've never noticed a build failure for lack of
a "CC" wrapper, despite there probably never having been such a
wrapper.

Given that jperkin's SmartOS bulk build with this change produced
results similar to last week's non-cwrappers trunk build, we can
conclude that across platforms, the need for a "CC" wrapper is very
limited and we needn't try (and sometimes fail) to create one by
default.

(schmonz)