internal/web3ext: remove the legacy backtraceAt method (ethereum#31783)

The function `BacktraceAt` has been removed in ethereum#28187 . But the API
end-point `debug_backtraceAt` is not removed from the file
`internal/web3ext/web3ext.go`.

Author: JukLee0ira

internal/web3ext/web3ext.go

@@ -247,11 +247,6 @@ web3._extend({
 			call: 'debug_vmodule',
 			params: 1
 		}),
-		new web3._extend.Method({
-			name: 'backtraceAt',
-			call: 'debug_backtraceAt',
-			params: 1,
-		}),
 		new web3._extend.Method({
 			name: 'stacks',
 			call: 'debug_stacks',