Remove :title attribute from disabled buttons to prevent un-styled tooltips

While adding the gitlab-ui tooltips to some buttons (gitlab!46922 (merged)) I spotted that disabled buttons do not display the sleek gitlab-ui tooltips, but revert to the "out of the box" tooltips;

Enabled;

Disabled;

I'm not sure quite how much overlap there is, but I did find this issue which may fix this; #926 (closed)

/cc @oregand @tauriedavis