Remove use of unsupported source key in patch spec

[Flamefire]

• The easyconfig parser doesn't allow this key
• The documentation doesn't mention it
• No useage in easyblocks or framework are found
• finalpath of all but the first source is builddir, see Correctly detect final path when extracting multiple tarballs  #4922

So just remove this code.

[boegel]

Makes sense, since trying to use source key in patches entry leads to clear error:

For example with:

patches = [{'source': 0, 'name': 'test.patch'}]

we get:

ERROR: ... Wrong patch spec '{'source': 0, 'name': 'test.patch'}', use of unknown key source in dict (valid keys are ['name', 'copy', 'level', 'sourcepath', 'alt_location', 'opts'])