Remove obsolete code (Fixes #20)

[SAK917]

Address change requested in Issue #20 by making the following changes:

Refactor writeto_then_readfrom() to be consistent with the other methods in the class by now returning self_tca_i2c.writeto_then_readfrom()

Remove obsolete code in writeto_then_readfrom() as per @jepler in Issue #20

_I did test the changes to the extent I could (i.e., no errors at runtime and able to instantiate an adafruittca9548a.TCA9548A object in code), but I do not have a TCA9548A board to physically test.

[SAK917]

I saw that this Issue was flagged as a Good First Issue that nobody had tackled yet and thought I would try to help out. Please don't hold back on suggestions or best practices, I am trying to learn!

[ladyada]

looks good! i dont think we need the return tho :)

[SAK917]

Thanks @ladyada, and sorry if I am missing something but if you look at all the code in this module, the other methods in this class (e.g., writeto(), unlock(), etc.) return the associated method. Is there something unique with this method that I modified that makes it different from the others? I am happy to remove the return if it is unneeded, but I just wanted to double check.

[ladyada]

imma defer to @tannewt on stylistic decision on whether to return. i feel like the original code did not return so replacement shouldnt?

[SAK917]

I am happy to defer to whatever you and @tannewt feel is correct. Just let me know and I will make the changes and push to the PR.