pwb.py: Simplify the import system
pwb.py: Simplify the import system

- Use `import pywikibot as pwb` instead of `pwb = pywikibot` in the else
  clause of the try statement.
- There is no need to call tryimport_pwb in main() because it's already
  been imported earlier.
- There is no need to call tryimport_pwb in run_python_file(), just make
  sure it is imported before calling it.
- No need for remove_modules() function. AFAICT, none of the
  config-dependant modules can be loaded successfully when the config file
  is not present. Removing and reimporting others is of no use.
- Remove the tryimport_pwb() as is it no longer used.

Change-Id: I49d728e08edd14edb5b4203d46349f39c1b75dc3

wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core