Ignore "Checking" quickfix line

Remove the line starting with `Checking` output by `cargo check` from
the quickfix buffer.

Author: casey

compiler/cargo.vim

@@ -34,6 +34,7 @@ augroup END
 " Ignore general cargo progress messages
 CompilerSet errorformat+=
             \%-G%\\s%#Downloading%.%#,
+            \%-G%\\s%#Checking%.%#,
             \%-G%\\s%#Compiling%.%#,
             \%-G%\\s%#Finished%.%#,
             \%-G%\\s%#error:\ Could\ not\ compile\ %.%#,