chore: remove btoa fallback (#8783)

Since we're targeting NodeJS >16 we don't need any Buffer fallback as it's now built in globally from that version onwards.

packages/svelte/src/compiler/utils/mapped_code.js

@@ -266,9 +266,6 @@ export function combine_sourcemaps(filename, sourcemap_list) {
 	if (!map.sources.length) map.sources = [filename];
 	return map;
 }
-// browser vs node.js
-const b64enc = typeof btoa == 'function' ? btoa : (b) => Buffer.from(b).toString('base64');
-const b64dec = typeof atob == 'function' ? atob : (a) => Buffer.from(a, 'base64').toString();
 
 /**
  * @param {string} filename
@@ -295,7 +292,7 @@ export function apply_preprocessor_sourcemap(filename, svelte_map, preprocessor_
 		toUrl: {
 			enumerable: false,
 			value: function toUrl() {
-				return 'data:application/json;charset=utf-8;base64,' + b64enc(this.toString());
+				return 'data:application/json;charset=utf-8;base64,' + btoa(this.toString());
 			}
 		}
 	});
@@ -339,7 +336,7 @@ export function parse_attached_sourcemap(processed, tag_name) {
 				// ignore attached sourcemap
 				return '';
 			}
-			processed.map = b64dec(map_data); // use attached sourcemap
+			processed.map = atob(map_data); // use attached sourcemap
 			return ''; // remove from processed.code
 		}
 		// sourceMappingURL is path or URL