logging: remove incorrect beta notice
logging: remove incorrect beta notice

We advertise this library as being stable in the README.

Change-Id: Ifb891c25ccb7232e334c7708aceee15bea7db4a5
Reviewed-on: https://code-review.googlesource.com/c/36231
Reviewed-by: Eno Compton <ad9228447a2ef39a5712f4b80b45071fa843fa0d@google.com>
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go