mb: Fix explicit android_{high,low}_end configs

This was leftover from the migration to change the default value for
is_high_end_android to be based on whether target_cpu was arm64
(default target_cpu == arm64 now matches is_high_end_android == true).

There is no longer a need to explicitly set `is_high_end_android = true`
in mb.py. And only arm64 configs that don't use PGO at all should
include `android_low_end`, there is now only two such config left. One
of which should be removed in a subsequent CL
(official_reclient_minimal_symbols_android_arm64_pgo) since there is no
point in testing a PGO config without high end, as PGO gets turned off
when it is not high end.

The rest of the `android_{high,low}_end` configs have been removed. This
should ensure that android-arm64-pgo (which should be high end) is now
correctly using the default instead of explicitly setting it to false.

Bug: 347023925, 41488915
Change-Id: I9654ce656ef430ac60a01ac01824bb2bada32382
Fixed: 347023925
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5630236
Auto-Submit: Peter Wen <wnwen@chromium.org>
Commit-Queue: Haiyang Pan <hypan@google.com>
Reviewed-by: Haiyang Pan <hypan@google.com>
Commit-Queue: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1314766}