build, test, doc: Temporarily remove Android-related stuff

Previously, our Android builds were geared towards generating APKs,
which relied on Qt. However, after migrating to C++20, compiling for
Android became unfeasible due to Qt 5.15's compatibility limitations
with NDK only up to r25, which includes an outdated embedded libc++.

All removed stuff will be reinstated after migrating the build system to
CMake and upgrading Qt to version 6.x."

doc/README.md

@@ -45,7 +45,6 @@ The following are developer notes on how to build Bitcoin Core on your native pl
 - [FreeBSD Build Notes](build-freebsd.md)
 - [OpenBSD Build Notes](build-openbsd.md)
 - [NetBSD Build Notes](build-netbsd.md)
-- [Android Build Notes](build-android.md)
 
 Development
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