Remove the 'requirements.txt' syntax

This is needed because the syntax is only available under GPL v3.0,
which is not compatible with `bat`s license.

assets/syntaxes/02_Extra/requirementstxt.sublime-syntax

@@ -1,19 +0,0 @@
-%YAML 1.2
----
-# http://www.sublimetext.com/docs/3/syntax.html
-name: requirements.txt
-file_extensions:
-  - requirements.txt
-scope: source.requirementstxt
-contexts:
-  main:
-    - match: (#).*$\n?
-      scope: comment.line.requirementstxt
-      captures:
-        1: punctuation.definition.comment.requirementstxt
-    - match: '(?i)^[a-z0-9_\-\.]+'
-      scope: string.package_name.requirementstxt
-    - match: (?i)==|<|<=|>=|>
-      scope: keyword.operator.logical.punctuation.requirementstxt
-    - match: '(?i)\d+\.[\da-z\-_\.]*'
-      scope: constant.numeric.verions.requirementstxt