Remove support for specialized treatment of form feeds

This is a very intentional change. Previously form feeds were handled very
poorly and sometimes where not counted as indentation. This obviously makes
sense. But at the same time indentation is very tricky to deal with (both for
editors and parso).

Especially in the diff parser this led to a lot of very weird issues. The
decision probably makes sense since:

1. Almost nobody uses form feeds in the first place.
2. People that use form feeds like Barry Warsaw often put a newline ater them.
   (e.g Python's email.__init__)
3. If you write an editor you want to be able to identify a unicode character
   with a clear line/column. This would not be the case if form feeds were just
   ignored when counting.

Form feeds will still work in Jedi, will not cause parse errors and in general
you should be fine using them. It might just cause Jedi to count them as
indentation **if** you use it like '\f  foo()'. This is however confusing for
most editors anyway. It leads to a weird display e.g. in VIM, even if it's
perfectly valid code in Python.

Since parso is a code analysis parser and not the languages parser I think it's
fine to ignore this edge case.

test/test_diff_parser.py

@@ -1406,3 +1406,24 @@ def test_error_dedent_in_function(differ):
         ''')
     differ.initialize(code1)
     differ.parse(code2, parsers=ANY, copies=ANY, expect_error_leaves=True)
+
+
+def test_x(differ):
+    code1 = dedent('''\
+        @bla
+        async def foo():
+            1
+            yield from []
+            return
+            return ''
+        ''')
+    code2 = dedent('''\
+        @bla
+        async def foo():
+            1
+        \x0cimport 
+            return
+            return ''
+        ''')
+    differ.initialize(code1)
+    differ.parse(code2, parsers=ANY, copies=ANY, expect_error_leaves=True)