Deletes header files and BUILD targets for SpdyUnsafeArena, which is no longer used.
Deletes header files and BUILD targets for SpdyUnsafeArena, which is no longer used.

gfe-relnote: n/a (deletes unused code/BUILD targets)
PiperOrigin-RevId: 284816046
Change-Id: I69f074a61077fc8396448c2524ea5387c57a46ab

google/quiche,google/quiche,google/quiche,google/quiche