Slight improvements in Mbed TLS shared heap helpers

This patch, firstly, makes the error messages consistent to how printed
strings are usually formatted. Secondly, it removes an unnecessary #if
directive.

Change-Id: Idbb8ef0070562634766b683ac65f8160c9d109e6
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>