Outstanding-loan error message does not follow style guide

[jruderman]

"assigning to mutable vec content prohibited due to outstanding loan"

"Prohibited" should be "disallowed" according to https://github.com/mozilla/rust/wiki/Note-style-guide. (Or "not allowed", but "X not allowed due to Y" is easy to misread.)

[lkuper]

I was the one who added that point in the style guide (because both "not allowed" and "not permitted" were in use and I wanted to pick one, and "not allowed" seemed more popular in the unscientific poll I took). But I hadn't thought about "prohibited" -- that's a third possibility. I'm not sure we can in good faith tell people to stop using "prohibited" and instead use "disallowed"/"not allowed". Maybe none of this matters?

[lkuper]

Yeah, I now think that that was a silly thing for me to have put in the style guide. I removed it and am closing this.