[ios] Deleted unused source unit tests

Deleted MGLSourceTests, which has been subsumed by MGLStyleTests and is no longer part of any test target.

platform/ios/test/MGLSourceTests.m

@@ -1,24 +0,0 @@
-#import "MGLMapViewTests.h"
-
-@interface MGLSourceTests : MGLMapViewTests
-
-@end
-
-@implementation MGLSourceTests
-
-- (void)testDuplicateSources {
-    MGLVectorSource *source1 = [[MGLVectorSource alloc] initWithIdentifier:@"my-source" URL:[NSURL URLWithString:@"mapbox://mapbox.mapbox-terrain-v2"]];
-    MGLVectorSource *source2 = [[MGLVectorSource alloc] initWithIdentifier:@"my-source" URL:[NSURL URLWithString:@"mapbox://mapbox.mapbox-terrain-v2"]];
-
-    [self.mapView.style addSource: source1];
-
-    @try {
-        [self.mapView.style addSource: source2];
-        XCTFail(@"Should not have reached this point");
-    }
-    @catch (NSException *e) {
-        XCTAssertNotNil(e, @"Should have thrown an exception");
-    }
-}
-
-@end