imgact_elf.c: style, remove unnecessary casts

Remove unnecessary type casts and redundant brackets.
No functional changes.

Suggested By: kib
Reviewed By: kib
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D30841

(cherry picked from commit 61b4c62718093fc3a30041bb7fa4d31c7b6c31b2)