scripts/kallsyms: remove redundant is_arm_mapping_symbol()

Since commit 6f00df24

 ("[PATCH] Strip local symbols from kallsyms"),
all symbols starting '$' are ignored.

is_arm_mapping_symbol() particularly ignores $a, $t, etc. but it is
redundant.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>