remove -mdspr2

remove -mdspr2

I set -mdspr2 unconditionally on 32-bit MIPS builds only to not have to
deal with setting it conditionally on the source files specifically
requiring those instructions.

Since then, I've deleted all the source files specifically requiring
-mdspr2 (and -mdsp).  No one understood them and they were only
accelerating really uninteresting cases, like 565 blits.

So, we haven't needed -mdspr2 anywhere for a while, and removing it
will unblock running these builds on the CI20 dev boards, which
support neither -mdsp nor -mdsp2.

BUG=skia:1437

CQ_INCLUDE_TRYBOTS=skia.primary:Build-Ubuntu-Clang-mipsel-Debug-Android,Build-Ubuntu-Clang-mipsel-Release-Android

Change-Id: Idda56fd8bb37ba064b78b4cddd8878b37dfe9502
Reviewed-on: https://skia-review.googlesource.com/11284
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>