Front end: remove getFunctionTypeForImplicitCall (and unnecessary invocations).

The method `InferenceVisitorBase.getFunctionTypeForImplicitCall` was
only being in one circumstance: when a method invocation resolves to a
field or getter which needs a runtime covariance check, it was being
used to determine what type should be used for the covariance
check. Its effect was to either return the type unchanged (in the case
where it was already a function type) or, in the case where the type
was an interface, to look up the type of the interface's `.call`
method.

But this is not the behavior we want. For a covariance check, the
right thing to do is simply to verify that the result of the getter
invocation satisfies the getter's static type (with appropriate
substitutions). It doesn't matter whether the type in question is an
interface type, a function type, or some other type.

Fixes https://github.com/dart-lang/sdk/issues/53089.

Change-Id: I064bea0fdf0ba3695f6ce51e9b84cc1421061a2e
Bug: https://github.com/dart-lang/sdk/issues/53089
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/317360
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>