test(coverage): disable text coverage reporter

The txt coverage reporter generates a lot of output that makes it hard
to see the actual test results without scrolling up a lot first.
We already have the html reporter setup and an npm script to enable
easy viewing of the data (`yarn coverage`) so the text reporter isn't
adding anything useful.

package.json

@@ -172,7 +172,6 @@
     "coverageReporters": [
       "json",
       "lcov",
-      "text",
       "html"
     ],
     "moduleNameMapper": {