libpng: Revert "libpng: add symlink to debug library" as unnecessary

libpng: Revert "libpng: add symlink to debug library" as unnecessary

This reverts commit 5313dd9be from PR #25324 , related to #25323 and
https://github.com/openwrt/openwrt/pull/16899

The additional symlink libpng16.so -> libpng16d.so is now unnecessary as
OpenWrt main repo PR https://github.com/openwrt/openwrt/pull/18709
commit https://github.com/openwrt/openwrt/commit/703e7d2d5b58e68528cb5fadfc1185bd3904b3d9
changed the cmake build type from 'Debug' to 'RelWithDebInfo', which
causes libpng .so to be compiled with the normal name libpng16.so
(instead of the debug-styled libpng16d.so).

Signed-off-by: Hannu Nyman <redacted>