device/testing/linux-*: remove unused compiler-gcc6.h files
These haven't been used since converting the APKBUILDs to use devicepkg-dev, so remove them.
Based on !1695 (merged)
Addresses #853 (closed)
These haven't been used since converting the APKBUILDs to use devicepkg-dev, so remove them.
Based on !1695 (merged)
Addresses #853 (closed)