Remove stale incremental-mode caveat from warn_unused_configs docs (#21899)

Since commit d3de4a4 (#20801), `warn_unused_configs` works in
incremental mode, so the docs no longer need to say that
`--no-incremental` / `incremental = False` is required.

Author: brianhelba

docs/source/command_line.rst

@@ -127,7 +127,6 @@ Config file
 
     This flag makes mypy warn about unused ``[mypy-<pattern>]`` config
     file sections.
-    (This requires turning off incremental mode using :option:`--no-incremental`.)
 
 
 .. _import-discovery:

docs/source/config_file.rst

@@ -1211,7 +1211,6 @@ These options may only be set in the global section (``[mypy]``).
 
     Warns about per-module sections in the config file that do not
     match any files processed when invoking mypy.
-    (This requires turning off incremental mode using :confval:`incremental = False <incremental>`.)
 
 .. confval:: verbosity