style: Remove redundant bound

[caspermeijn]

Remove where bound when the same bound is already specified.

This solves clippy warnings like this:

warning: bound is defined in more than one place
   --> src/query/grammar.rs:169:21
    |
169 | pub fn mutation<'a, T: Text<'a>>(
    |                     ^
...
173 |     T: Text<'a>,
    |     ^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#multiple_bound_locations

[LegNeato]

Thanks!