Remove apex10000_private variants

When a library is included in two APEXes whose platform_apis settings
are different, two apex variants of the library is created: apex1000 and
apex1000_private.

This change was introduced with ag/15061306, especially by the commit
[1].

However, that part should be reverted because it actually creates
unnecessary variants. It's unnecessary because the two variants of the
library are compiled (excluding the linking) exactly the same. If a
private symbol of its dependency was actually used when compiling the
apex1000_private variant, then the other apex1000 variant wouldn't have
been built because that private symbol must have caused a linkage error.

[1] https://googleplex-android-review.git.corp.google.com/c/platform/build/soong/+/15061306/2..4/android/apex.go#b527).

Bug: 228785792
Test: m
Change-Id: Id58d3e98a51de5e628ca72ef86e9cd11b0ee8971