pw_function: Delete copy constructor
pw_function: Delete copy constructor

pw::Function should be move only so delete copy constructor and copy
assignment operator.

Change-Id: I9a2cecf9f18064bc2d37bd5d2c91d8209ddc92f1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/92220
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Commit-Queue: Austin Foxley <1af965fecf930e25ddc640dc59fb3dfab8e1466a@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed