Remove old Strawberry Perl workaround

This removal is not because the underlying issue is fixed. Strawberry
Perl seems uninterested in fixing this.[1]

However, our workaround seems to have had some other consequences
(https://crbug.com/525786169), and Perl is less hard of a requirement
for us now. You still need Perl to modify parts of BoringSSL, but
building BoringSSL no longer needs Perl.

Given that, it is probably simplest to just stay out of the business of
overwriting CMake, even if CMake gets it wrong as as result of
Strawberry Perl's misbehavior. Instead, we update the documentation to:

- Remove mention of ActiveState Perl or MSYS Perl. ActiveState Perl
  seems to require an account to download now[2]. Despite the
  misbehavior, perl.org points to Strawberry Perl, so let's stick to
  just one recommended configuration.

- Recommend the portable zip of Strawberry Perl, not the MSI installer.
  That doesn't mess with PATH.

Otherwise, leave fixing the environment to the person setting up the
environment. Strawberry Perl's misbehavior seems to have all kinds of
side effects, so likely our workaround wasn't sufficient for more
complicated setups anyway. E.g. see [3] and [4]. Though [3] is a little
worrisome. It's possible some folks' CIs will need to start passing
CMAKE_IGNORE_PATH.

Update-Note: BoringSSL no longer works around Strawberry Perl for you.
Let us know if it turns out your setup was relying on it.

[1] https://github.com/StrawberryPerl/Perl-Dist-Strawberry/issues/11
[2] https://www.reddit.com/r/perl/comments/d9gctx/activeperl_is_dead_to_me/
[3] https://github.com/actions/runner-images/issues/6627
[4] https://gitlab.kitware.com/cmake/cmake/-/work_items/23975

Fixed: 525786169
Change-Id: Ia3c9e3885e095af6fb9df15bf49a00a163d8dd51
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/97767
Auto-Submit: David Benjamin <davidben@google.com>
Presubmit-BoringSSL-Verified: boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>