Remove FLAG_SPLIT_TOUCH from launcher code

The splitting behaviour is now permanent, so this flag is deprecated and
it's no longer necessary to set it.

Bug: 239934827
Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Flag: NONE removing flag
Change-Id: Ie7dafaeffdb6f5854a61e6ece0217d8bf5e496af