Remove redundant RIG_PTT_SERIAL_CAT enum from ptt_t

The Icom IC-7100 back end added this enum instead of using the extant
RIG_PTT_RIG enum.  Also the rig_get_ptt() implementation didn't handle
it and errored out.

Note: RIG_PTT_SERIAL_CAT was added in commit
e9ee671149ad41a4fe000a40414d9d6ee12c60a4  - N0NB