Remove drake-distro from example READMEs

[jwnimmer-tri]

This also makes valkyrie_pd_ff_controller executable again -- we'd been compiling it as a library ever since we switched to Bazel. As it turns out, its a main program split into header + cc.

Relates #6996.

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This change is 

[jwnimmer-tri]

+@amcastro-tri could I ask you to feature-review this cleanup, please?

[amcastro-tri]

Thanks for the cleanup @jwnimmer-tri.

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Reviewed 5 of 5 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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systems/sensors/test/accelerometer_test/README.md, line 3 at r1 (raw file):

To run a demo of an accelerometer attached to a pendulum:

# When building Drake using Bazel

BTW this comment is probably not needed anymore.

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Comments from Reviewable

[amcastro-tri]

+@sammy-tri for platform review please.

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Review status: all files reviewed at latest revision, 1 unresolved discussion.

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[jwnimmer-tri]

Review status: all files reviewed at latest revision, 1 unresolved discussion.

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systems/sensors/test/accelerometer_test/README.md, line 3 at r1 (raw file):

Previously, amcastro-tri (Alejandro Castro) wrote…

BTW this comment is probably not needed anymore.

Actually users can and do still use CMake builds. https://github.com/RobotLocomotion/drake/blob/master/CMakeLists.txt

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Comments from Reviewable

[sammy-tri]

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Reviewed 5 of 5 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable