scripts/lib/kdoc/kdoc_parser.py: remove a python 3.9 dependency

str.removesuffix() was added on Python 3.9, but rstrip()
actually does the same thing, as we just want to remove a single
character. It is also shorter.

So, use it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/f64cc4adef107ada26da4bfb7e4b7002dd783173.1744106242.git.mchehab+huawei@kernel.org

Author: mchehab

scripts/lib/kdoc/kdoc_parser.py

@@ -1641,7 +1641,7 @@ def run(self):
                     # Group continuation lines on prototypes
                     if self.state == self.STATE_PROTO:
                         if line.endswith("\\"):
-                            prev += line.removesuffix("\\")
+                            prev += line.rstrip("\\")
                             cont = True
 
                             if not prev_ln: