Remove some obsolete button styles from _common.pcss (#10548)

* Remove obsolete style rules - `mx_linkButton`

The block was added with fbab8ce for CreateKeyBackupDialog.js. It has been replaced with AccessibleButton element (see: onSkipPassPhraseClick on L283 and changeText on L314-316). The last function which had used mx_linkButton (_onOptOutClick) has been removed.

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Remove obsolete style rules - `mx_textButton`

mx_textButton has been replaced with AccessibleButton (with kind="link"). There is also not an element which uses the class.

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

---------

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

res/css/_common.pcss

@@ -657,11 +657,6 @@ legend {
     margin: auto;
 }
 
-.mx_linkButton {
-    cursor: pointer;
-    color: $accent;
-}
-
 .mx_TextInputDialog_label {
     text-align: left;
     padding-bottom: 12px;
@@ -676,16 +671,6 @@ legend {
     background-color: $background;
 }
 
-@define-mixin mx_DialogButton_small {
-    @mixin mx_DialogButton;
-    font-size: $font-15px;
-    padding: 0px 1.5em 0px 1.5em;
-}
-
-.mx_textButton {
-    @mixin mx_DialogButton_small;
-}
-
 .mx_button_row {
     margin-top: 69px;
 }