[CMAKE] Remove extra -fPIE flag
[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

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