s390/con3270: rewrite command line recalling

Command line recalling is the last user of the 3270 custom malloc()
like allocator. Remove this dependency by using a statically allocated
buffer for the saved command lines, and also remove the allocator.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>