staging: rtl8192e: Remove unused rf_set_sens variable

staging: rtl8192e: Remove unused rf_set_sens variable

The rf_set_sens variable is declared but never set, so it is always NULL.
This commit cleans up the unused rf_set_sens variable and removes the
associated code that relied on it.

Specifically, the following changes have been made:
- Removed the `range->sensitivity` assignment, which was never used.
- Removed the sensitivity level get implementation, which always returns
  an error code.
- Removed the sensitivity level set implementation, which always returns
  an error code.
- Removed the `rf_set_sens` variable.

Suggested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Signed-off-by: Yogesh Hegde <yogi.kernel@gmail.com>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Link: https://lore.kernel.org/r/5a9767fab76b1836ea7881994ffb3593c1ab12bf.1682358035.git.yogi.kernel@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>