chore: remove abbreviations linted in mathlib (#764)

C.f.
#522 (comment)

Author: mhuisi

lean4-unicode-input/src/abbreviations.json

@@ -343,7 +343,6 @@
     "aa": "å",
     "ae": "æ",
     "austral": "₳",
-    "afghani": "؋",
     "amalg": "∐",
     "average": "⨍",
     "-int": "⨍",
@@ -1026,7 +1025,6 @@
     "caution": "☡",
     "cap": "∩",
     "qed": "∎",
-    "quad": " ",
     "quot": "⧸",
     "bigsolidus": "⧸",
     "/": "⧸",
@@ -1385,7 +1383,6 @@
     "_(": "₍",
     "_=": "₌",
     "_+": "₊",
-    "_--": "̲",
     "_-": "₋",
     "!!": "‼",
     "!?": "⁉",