Remove PKCS1_CHECK

The code guarded by PKCS1_CHECK appears to be unhelpful, and the guard
is explicitly undefined in ssl_locl.h Remove both.

Change-Id: I3cd45a744a8f35b02181b1e48fd1ef11af5e6f4a
Reviewed-on: https://boringssl-review.googlesource.com/1383
Reviewed-by: Adam Langley <agl@google.com>