src/buildall.bash: remove mobile filter

Mobile targets are not supported by misc-compile trybots, as tracked in
golang.org/issue/25963, and need to be filtered out. The buildall.bash
script was created in CL 9438, back when it was a single all-compile
builder, and it was easier to filter out mobile targets in the script
than to come up with a pattern that matches all non-mobile targets.

As of CL 254740, all mobile targets (Android and iOS) have unique GOOS
values. That makes it it easy to filter them out in x/build/dashboard.
This was done in CL 258057. As a result, it's now viable to simplify
this script and perform all misc-compile target selection in x/build,
rather than having it spread it across two places.

Also, as of CL 10750, the all-compile builder has turned into multiple
misc-compile builders, so update the script description accordingly.

Updates #41610.

Change-Id: I1e33260ac18cf0a70bb68cd8e3db5587100c7e87
Reviewed-on: https://go-review.googlesource.com/c/go/+/257962


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>