Remove OS_ANDROID check.

RE2 does not have an OS_ANDROID define, so there is no point in checking
it.

It serves only downstream in Chromium, where OS_ANDROID is actually
defined, but they maintain their own fork
(https://chromium.googlesource.com/chromium/src/+/master/third_party/re2),
so this is not needed here.

This was introduced in
https://github.com/google/re2/commit/cc56ba02d9d2bdafa614ad5ebf564dde287625bb

Change-Id: I7f97520bb4a50f9b3f2cb179f20d75411f00526c
Reviewed-on: https://code-review.googlesource.com/2441
Reviewed-by: Paul Wankadia <junyer@google.com>