src/rxosd/Makefile.in: avoid infinite recursion in CC variable

src/rxosd/Makefile sets CC=$(MT_CC).
src/config/Makefile.config sets MT_CC=$(CC).
Since both are recursive-expansion variables, this causes an infinite
loop, and make complains about it:

Makefile:25: *** Recursive variable `CC' references itself (eventually).  Stop.

This patch avoids setting CC in src/rxosd/Makefile.in altogether, since
it does not appear to be necessary, as suggested by Jeffrey and Derrick.

Change-Id: Ieaa5c5ed21d83629ea713a745e76d17ac7f76ec1
Reviewed-on: http://gerrit.openafs.org/9336
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>