command-list.txt: drop the "common" tag

command-list.txt: drop the "common" tag

command-list.sh, retired in the previous patch, was the only
consumer of the "common" tag, so drop this now-unnecessary
attribute.

before:
    git-add          mainporcelain        common worktree

after:
    git-add          mainporcelain        worktree

Helped-by: Eric Sunshine <redacted>
Signed-off-by: Sébastien Guimmara <redacted>
Reviewed-by: Eric Sunshine <redacted>
Signed-off-by: Junio C Hamano <redacted>