disas/arm: Fix warnings caused by missing 'static' attribute

disas/arm: Fix warnings caused by missing 'static' attribute

Warnings from the Sparse static analysis tool:

disas/arm.c:1552:15: warning:
 symbol 'last_type' was not declared. Should it be static?
disas/arm.c:1553:5: warning:
 symbol 'last_mapping_sym' was not declared. Should it be static?
disas/arm.c:1554:9: warning:
 symbol 'last_mapping_addr' was not declared. Should it be static?

Instead of adding 'static', the unused variables and the unused code which
refers to those variables (which was deactivated a long time ago in
commit 4b0f1a8b) are removed.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>