[theme] Remove theme.mixins.gutters

[joshwooding]

Breaking change

The abstraction hasn't proven to be used frequently enough to be valuable.

-theme.mixins.gutters(),
+paddingLeft: theme.spacing(2),
+paddingRight: theme.spacing(2),
+[theme.breakpoints.up('sm')]: {
+  paddingLeft: theme.spacing(3),
+  paddingRight: theme.spacing(3),
+},

• I have followed (at least) the PR section of the contributing guide.

An easy one to cross off.

Part of #20012

[mui-pr-bot]

Details of bundle changes

Generated by 🚫 dangerJS against d5e3c71

[oliviertassinari]

@joshwooding Don't forget about the deprecation on v4 :)