Remove old config file for SENTRY5

This is an older broadcom part that implements the mips32 ISA. 32-bit
FreeBSD/mips now requires mips32r2, so retire this config. Most of the
broadcom port is shared with newer ports, so what little code may be
unique to this part has not been GC'd at this time.

Discussed on: freebsd-mips@
Differential Revision: https://reviews.freebsd.org/D18543