remove Single button toggle example

[amitport]

According to https://material.io/guidelines/components/buttons.html#buttons-toggle-buttons:

"
Toggle button requirements:

Have at least three toggle buttons in a group
"

a single non-icon button toggle shown in the example (https://github.com/angular/material2/blob/20a23f16e8ac1ced29172709cc837fa500b6377b/src/material-examples/button-toggle-overview/button-toggle-overview-example.html)
is not recommended (and does not communicate that the button can be toggled properly IMHO)

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