Remove execute-only memory related code.

The kernel no longer enforces XOM layouts, and the build system no
longer generates XOM binaries. As a result, the zygote code to mark XOM
sections of memory readable for apps with targetSdkVersion<Q.

Bug: 147300048
Test: m -j
Change-Id: I0abb0abb54f8f5a538da9194a900e6a64e574bfe