rustfmt: Remove required_version

The `required_version` config option adds very little value and is
annoying to maintain because it has to be updated everytime a new
version of `clippy` drops.

Author: tcharding

rustfmt.toml

@@ -68,7 +68,6 @@ use_field_init_shorthand = false
 force_explicit_abi = true
 condense_wildcard_suffixes = false
 color = "Auto"
-required_version = "1.5.1"
 unstable_features = false
 disable_all_formatting = false
 skip_children = false