[gdb/testsuite] Remove use of then keyword some more

Find uses of the then keyword:
...
$ find gdb/testsuite/ -type f -name *.exp* | xargs grep "if.*then {"
...
and remove them.

See also commit d4c4542312c ("gdb/testsuite: remove use of then keyword from
library files") and related commits.

Tested on aarch64-linux.