python: remove redundant is_string_type

python: remove redundant is_string_type

In commit 72f7994 (python interface: support 'char *' and 'const *'
return types, Thu Jun 30 16:22:34 2016), the is_string_type() method was
added accidentally despite is_string() already providing the very
functionality.  Even worse, is_string_type() has a bug where a "char"
interpreted as boolean true and it also contains a seemingly unnecessary
call to isFunctionType().  Remove the redundant function definition and
switch the two users to is_string().

This change does not change the output of isl.py.

Reported-by: Sven Verdoolaege <skimo@kotnet.org>
Signed-off-by: Tobias Grosser <tobias@grosser.es>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>