Remove newline from start of deprecation warnings

This breaks the summary from Pytest when warnings are errors, as then
you just get a bunch of:
```
FAILED lib/matplotlib/tests/test_getattr.py::test_getattr[matplotlib.afm] - matplotlib._api.deprecation.MatplotlibDeprecationWarning:
```
with no real context as to the problem.

Also, as noted at the original PR [1], it makes setting `filterwarnings`
more complicated.

[1] matplotlib#11224 (comment)

Author: QuLogic

lib/matplotlib/_api/deprecation.py

@@ -32,7 +32,7 @@ def _generate_deprecation_warning(
         removal = f"in {removal}" if removal else "two minor releases later"
     if not message:
         message = (
-            ("\nThe %(name)s %(obj_type)s" if obj_type else "%(name)s")
+            ("The %(name)s %(obj_type)s" if obj_type else "%(name)s")
             + (" will be deprecated in a future version"
                if pending else
                (" was deprecated in Matplotlib %(since)s"