styles: Undo calc(x + y) → calc(x - -y) workaround.

The bug this was working around does not affect our current toolchain,
as confirmed by grepping through the minified output.

(Also, this linter rule only matched calc(x + y) with two arguments
and we were already using calc($far_left_gutter_size + $left_col_size
+ 4px).)

Signed-off-by: Anders Kaseorg <anders@zulip.com>

tools/linter_lib/custom_check.py

@@ -498,10 +498,6 @@ bash_rules = RuleList(
 css_rules = RuleList(
     langs=['css', 'scss'],
     rules=[
-        {'pattern': r'calc\([^+]+\+[^+]+\)',
-         'description': "Avoid using calc with '+' operator. See #8403 : in CSS.",
-         'good_lines': ["width: calc(20% - -14px);"],
-         'bad_lines': ["width: calc(20% + 14px);"]},
         {'pattern': r'^[^:]*:\S[^:]*;$',
          'description': "Missing whitespace after : in CSS",
          'good_lines': ["background-color: white;", "text-size: 16px;"],