doc: syntax: remove syntax_checker.go from README.md

The patch is taken from:
micro-editor#2738

The mentioned file was already removed with the following commit:
micro-editor@fe3186b

The `micro` binary itself takes now care of validating the syntax definitions
and informs about possible issues.

Author: sertonix

runtime/syntax/README.md

@@ -6,11 +6,6 @@ Each yaml file specifies how to detect the filetype based on file extension or h
 In addition, a signature can be provided to help resolving ambiguities when multiple matching filetypes are detected.
 Then there are patterns and regions linked to highlight groups which tell micro how to highlight that filetype.
 
-Making your own syntax files is very simple. I recommend you check the file after you are finished with the
-[`syntax_checker.go`](./syntax_checker.go) program (located in this directory). Just place your yaml syntax
-file in the current directory and run `go run syntax_checker.go` and it will check every file. If there are no
-errors it will print `No issues!`.
-
 You can read more about how to write syntax files (and colorschemes) in the [colors](../help/colors.md) documentation.
 
 # Legacy '.micro' filetype