[fix] webapp.py - running without installation is not supported

Remove extension of the sys.path (aka PYTHONPATH).  Running instance directly
from repository's folder is a relict from the early beginning in
2014 (fd651083) and is no longer supported.

Since commit dd466297 was merged the command line 'searx-run' exists and should
be used.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>