Remove unused backup parser and old antlr4 dependency

Delete mdl/grammar/backup/ which contained a stale ANTLR 4.7.2
parser snapshot from the initial commit. Nothing imported it, but
it pulled in the old github.com/antlr/antlr4 module — causing an
unnecessary download on CI runners.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ako

go.mod

@@ -3,7 +3,6 @@ module github.com/mendixlabs/mxcli
 go 1.26.0
 
 require (
-	github.com/antlr/antlr4 v0.0.0-20210105192202-5c2b686f95e1
 	github.com/antlr4-go/antlr/v4 v4.13.1
 	github.com/chzyer/readline v1.5.1
 	github.com/jackc/pgx/v5 v5.8.0

go.sum

@@ -10,8 +10,6 @@ github.com/Azure/azure-sdk-for-go/sdk/security/keyvault/internal v1.2.0 h1:nCYfg
 github.com/Azure/azure-sdk-for-go/sdk/security/keyvault/internal v1.2.0/go.mod h1:ucUjca2JtSZboY8IoUqyQyuuXvwbMBVwFOm0vdQPNhA=
 github.com/AzureAD/microsoft-authentication-library-for-go v1.6.0 h1:XRzhVemXdgvJqCH0sFfrBUTnUJSBrBf7++ypk+twtRs=
 github.com/AzureAD/microsoft-authentication-library-for-go v1.6.0/go.mod h1:HKpQxkWaGLJ+D/5H8QRpyQXA1eKjxkFlOMwck5+33Jk=
-github.com/antlr/antlr4 v0.0.0-20210105192202-5c2b686f95e1 h1:9K5yytxEEQc4yIn6c1rvQD6qQilQn9mYIF7pXKPT8i4=
-github.com/antlr/antlr4 v0.0.0-20210105192202-5c2b686f95e1/go.mod h1:T7PbCXFs94rrTttyxjbyT5+/1V8T2TYDejxUfHJjw1Y=
 github.com/antlr4-go/antlr/v4 v4.13.1 h1:SqQKkuVZ+zWkMMNkjy5FZe5mr5WURWnlpmOuzYWrPrQ=
 github.com/antlr4-go/antlr/v4 v4.13.1/go.mod h1:GKmUxMtwp6ZgGwZSva4eWPC5mS6vUAmOABFgjdkM7Nw=
 github.com/benbjohnson/clock v1.1.0 h1:Q92kusRqC1XV2MjkWETPvjJVqKetz1OzxZB7mHJLju8=