tcg: Delete unused cpu_pc_from_tb()

No code uses the cpu_pc_from_tb() function. Delete from tricore and
arm which each provide an unused implementation. Update the comment
in tcg.h to reflect that this is obsoleted by synchronize_from_tb.

Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>