bootstrap: remove dead code

The workaround added by commit 04f442f755 (Workaround for short jump
tables on PA-RISC., 2014-11-04, v3.1.0-rc2~7^2) was only partially
removed by commit 6fdcf02aaf (bootstrap: remove dead branches,
2017-08-15, v3.10.0-rc1~250^2~2).  Remove the rest of it.

Also remove an unused check for Linux systems.