Remove the "image" storage class

This doesn't exist in WGSL, and we don't have a supported path to
implement this in MSL.

This was preventing "image" from being used as an identifier.

Change-Id: I724e46c866a4dd488f45fbc1215ef23a67355a78
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/78280
Auto-Submit: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: David Neto <dneto@google.com>

src/resolver/function_validation_test.cc

@@ -823,7 +823,6 @@ INSTANTIATE_TEST_SUITE_P(
                     TestParams{ast::StorageClass::kWorkgroup, true},
                     TestParams{ast::StorageClass::kUniformConstant, false},
                     TestParams{ast::StorageClass::kStorage, false},
-                    TestParams{ast::StorageClass::kImage, false},
                     TestParams{ast::StorageClass::kPrivate, true},
                     TestParams{ast::StorageClass::kFunction, true}));