Fix a build warning due to `documentation/Makefile` missing

This resolves the following warning when building the package:

```
warning: no files found matching 'documentation/Makefile'
```

MANIFEST.in

diff --git a/MANIFEST.in b/MANIFEST.in
index b79c1e7..8242644 100644
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -34,5 +34,3 @@
 include documentation/*.rst
 include documentation/pyserial.png
 include documentation/conf.py
-include documentation/Makefile
-