distutils.bbclass: only modify *.py file if it contains path to be removed

distutils.bbclass: only modify *.py file if it contains path to be removed

Currently sed command touches every single *.py file. This modifies the
timestamp of the file. All *.pyo files will be recompiled during the first
boot, because timestamp will not match. This should be only necessary if
sed command changes the file.

Signed-off-by: Radek Dostal <radek.dostal@streamunlimited.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>