style: Remove explicit lifetimes

[caspermeijn]

cargo clippy reports:

error: the following explicit lifetimes could be elided: 'a
  --> prost-types/src/conversions.rs:46:6
   |
46 | impl<'a> From<&'a str> for Value {
   |      ^^        ^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
   = note: `-D clippy::needless-lifetimes` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::needless_lifetimes)]`
help: elide the lifetimes
   |
46 - impl<'a> From<&'a str> for Value {
46 + impl From<&str> for Value {