Remove p!.

It's a cryptic macro that makes some things slightly more concise in
`PrettyPrinter`. E.g. if you declare `define_scope_printer!(p)` in a
scope you can then call `p! to get these transformations:
```
p!("foo");         -->  write!(p, "foo")?;
p!(print(ty));     -->  ty.print(p)?;
p!(method(args));  -->  p.method(args)?;
```
You can also chain calls, e.g.:
```
p!("foo", print(ty));  -->  write!(p, "foo")?; ty.print(p)?;
```
Ultimately this doesn't seem worth it. The macro definition is hard to
read, the call sites are hard to read, `define_scope_printer!` is pretty
gross, and the code size reductions are small. Tellingly, many normal
`write!` and `print` calls are sprinkled throughout the code, probably
because people have made modifications and didn't want to use or
understand how to use `p!`.

This commit removes it.

Author: nnethercote