stbt.FrameObject docstring: Remove extraneous links
stbt.FrameObject docstring: Remove extraneous links

It's confusing to link to the same page twice; and we already talk about
Page Objects in our (twice-linked) Object Repository page on the manual.

Note: We still keep the martinfowler.com link because it's used further
up in the caption of the PageObject diagram.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester