build-unit-test-docker: remove catch2

Based on searching the org in Github[1], there do not appear to be
anymore usages of Catch2.  Remove it from our Docker image.

1. https://github.com/search?q=org%3Aopenbmc+catch2&type=code

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I2cf89c533149266aa908a15bd825bfdbcd450c55
1 file changed