[CMAKE] Remove extra -fPIE flag

Summary:
Since cmake 3.8.2, the -fPIE flag is managed by cmake and added to the
compilation flags if POSITION_INDEPENDENT_CODE is ON. Adding it
manually is redundant at most, and error prone at worst.
Note that the -pie linker flag still needs to be added manually until
cmake 3.14+ is enforced.

Test Plan:

# You may need to clear your ccache
cmake -GNinja ..
ninja check-security

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4948