bootstrap/soenv: remove tcsh version of LinuxX86-64Env.Set

Previously we would have been writing out LinuxX86-64Env.Set and
LinuxX86-64Env.Set.sh which made my autocompletion useless. Simplify the
set_soenv.in to actually only produce 1 version which makes us require
bash as interactive build shell.
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>