_stbt.logging: Remove unused "--structured-logging" arg
_stbt.logging: Remove unused "--structured-logging" arg

This was added in 537f37e at the same time as "--save-trace", but
"--structured-logging" was never used anywhere.

LewisHaley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester