Use go:generate for generating cheatsheets

This has several benefits:
- it's less code
- we're using the same mechanism to generate all our auto-generated files, so if
  someone wants to add a new one, it's clear which pattern to follow
- we can re-generate all generated files with a single command
  ("go generate ./...", or "make generate")
- we only need a single check on CI to check that all files are up to date (see
  previous commit)

scripts/cheatsheet/main.go

@@ -1,27 +0,0 @@
-package main
-
-import (
-	"fmt"
-	"log"
-	"os"
-
-	"github.com/jesseduffield/lazygit/pkg/cheatsheet"
-)
-
-func main() {
-	if len(os.Args) < 2 {
-		log.Fatal("Please provide a command: one of 'generate', 'check'")
-	}
-
-	command := os.Args[1]
-
-	switch command {
-	case "generate":
-		cheatsheet.Generate()
-		fmt.Printf("\nGenerated cheatsheets in %s\n", cheatsheet.GetKeybindingsDir())
-	case "check":
-		cheatsheet.Check()
-	default:
-		log.Fatal("\nUnknown command. Expected one of 'generate', 'check'")
-	}
-}