[kokoro] Remove remaining wildcard Xcode version support. (#5312)
[kokoro] Remove remaining wildcard Xcode version support. (#5312)

This removes the remaining logic that supported running a kokoro job against every version of Xcode installed on the system.

The reason we are removing this logic is because running multiple instances of Xcode in sequence on a given machine increases the likelihood of flaky build failures in the Xcode toolchain (simulators, primarily).

All of our kokoro jobs already run against individual Xcode versions using the XCODE_VERSION environment variable.
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