Remove `METHOD` from `@Target` meta-annotation of `LengthOf`

It is surprising that the type annotation LengthOf is meta-annotated as

@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER, ElementType.METHOD})

Here is an excerpt from commit 71379bb7de:

-@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
+// Has target of METHOD so that it is stored as a declaration annotation and SameLen Checker can read it.
+@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER, ElementType.METHOD})
 public @interface LengthOf {

We have a FAQ that says not to make an annotation applicable to both type uses and declarations.

The METHOD target has surprising results.
For example, it means that when a source has only one occurrence of @LengthOf, its corresponding .class file has two occurrences.
A recent bug report says that this behavior is confusing.

I propose that we clean up the code (and help Shubham, who reported the issue I mentioned above) by doing 2 things:

1. Remove METHOD from LengthOf's @Target meta-annotation.
2. Remove the special-case handling of String.length in isLengthOfMethodInvocation.

Both items require changes to isLengthOfMethodInvocation.

Regarding item 1:

In
isLengthOfMethodInvocation
this line should look for a type, not a declaration, annotation:

        AnnotationMirror len = factory.getDeclAnnotation(ele, LengthOf.class);

Regarding item 2:

Remove the first clause from the implementation of isLengthOfMethodInvocation:

     public boolean isLengthOfMethodInvocation(ExecutableElement ele) {
        if (stringLength.equals(ele)) {
            // TODO: Why not just annotate String.length with @LengthOf and thus eliminate the
            // special case in this method's implementation?
            return true;
        }