[runtime] Remove obsolete Object::IsSpecFunction.

[runtime] Remove obsolete Object::IsSpecFunction.

We don't need Object::IsSpecFunction anymore, since it only checks for
JSFunction and JSFunctionProxy, but what you actually want to check for
(in case of accessors) is whether the target has a [[Call]] internal
method, which is exactly what Object::IsCallable does.

CQ_INCLUDE_TRYBOTS=tryserver.v8:v8_linux_layout_dbg,v8_linux_nosnap_dbg
R=rossberg@chromium.org
BUG=v8:4413
LOG=n

Review URL: https://codereview.chromium.org/1358403002

Cr-Commit-Position: refs/heads/master@{#30875}