bash-completion: dmesg: remove redundant completion for --buffer-size

bash-completion: dmesg: remove redundant completion for --buffer-size

The option --buffer-size already infers that its the optional argument
is a size value. In addition to that, when the option is completed the
user will have to delete the word 'size' from the command line and type
the desired value, which is not an ideal user experience.

Signed-off-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>