Uses @synthesize for touchController to avoid warning (#7670)
Uses @synthesize for touchController to avoid warning (#7670)

As touchController is a property defined in a class extension, it
is auto-synthesized (if enabled when compiling the file) because
there are no setters in the class @implementation.

However, since there is a setter in a category, two setters are
generated, thus causing the following linker warning:

  ld: warning: instance method 'touchController' in category from MDCThumbTrack.o overrides method from class in MDCThumbTrack.o

Instead remove the custom setter and uses @synthesize to ensure
that only one implementation of the setter exists.

Closes #7669.
material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios