remove custom Android logging, use syslog

As far as I can tell, syslog.h is fully supported for logging to Android's logcat. syslog.h is present as early as NDK r10e, if not earlier. It is also included on android-9 (2.3). My quick tests show syslog working fine on android-22 (5.1) through android-29 (10) emulators. android-24 (7.0) is currently the oldest version of Android supported by Google. syslog severity and identity tag are already mapped to the logcat versions.

@ahf is there some detail I'm missing? Were you having problems with syslog on Android?

Since the "android" logging option was never documented, I think it should just be removed, long with the add_android_log() code that was added in legacy/trac#24362 (moved) https://github.com/torproject/tor/commit/b0b8f7c30c8296344c77d06a24821f35b667e3dd

FYI, since syslog support is added about the same time as logcat support, legacy/trac#32036 (moved) is still an issue when using syslog with Android.