Remove redundant error message in tftf_initialise_timer()

tftf_cold_boot_main() - which calls tftf_initialise_timer() - already
prints an error message if the former return an error code.

Change-Id: I85d55ef8734ab3435afc34ac693109e24ebda661
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>