Android: remove dead Ant/Eclipse build scripts

The Ant build hasn't been runnable since the SDK dropped tools/ant/, so build.xml,
custom_rules.xml, ab-ant.sh, ant-build.bat, project.properties and
proguard-project.txt all go. Plus some orphans: buildassets.sh (nothing called it),
build.sh (still referred to "phoenix"), symx86.cmd (x86 isn't in APP_ABI) and
README.TXT (Eclipse import instructions).

ab.sh and ab.cmd no longer copy assets into android/assets - only the Ant/Eclipse
packaging ever read that directory. Gradle packages the repo-root assets/ directly
and ndk-build doesn't look at assets at all, so the stray copy only made it unclear
where the APK's assets come from. Noted how that actually works in docs/building.md.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

.gitignore

@@ -30,7 +30,6 @@ Windows/ipch
 Windows/bin-release
 Windows/win-version.h
 Windows/win-version-*.tmp
-android/assets
 ext/_Output
 android/lint.xml
 PPSSPPWindows64.exe.manifest
@@ -96,11 +95,6 @@ __testcompare.png
 vt_ref.bmp
 GameLogNotes.txt
 screenshots
-android/assets/lang
-android/assets/flash0
-android/assets/flash1
-android/assets/flash2
-android/assets/flash3
 font_atlas.zim.png
 asciifont_atlas.zim.png
 ppge_atlas.zim.png