Add/use sbbs_t::fremove() to remove files with logged error on failure

This should resolve CID 487088: Unchecked return value from library
as well as add some more instrumentation upon unexpected failures removing
files.

Consider: do some callers of this function need to know the success status
(return bool)? That then starts to get into the some callers check and some
don't situation (Coverity issue). For now, this is a void function.

Also, all remove failures are logged as errors. Consider possibly passing
a log level and calling lprintf() instead of errormsg(). errormsg() displays
a notice to the user, which maybe we don't want.