[lldb/Python] Unify PythonFormat & GetPythonValueFormatString (NFC)

This patch removes all occurences to GetPythonValueFormatString and
use the template specialization of PythonFormat structs instead.

Differential Revision: https://reviews.llvm.org/D134033



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>