Fix compilation warning.

Two issues at hand:
When doing PopScreen(false, false), do you call PopScreen(screen, bool) where screen = 0 or PopScreen(bool, bool)
Don't make PopScreen(void) virtual, so no virtual function is hiding the other.

Equivalent functionality is available via PopScreen(GetTopScreen)
Update ABI number following 474cdb1b88b606c729bb560a2b9065e99d15d7db
remove unused variable after last usage got removed in 4fc35ec91d

Signed-off-by: Jean-Yves Avenard <jyavenard@mythtv.org>