go: remove target directory before copying to it

Otherwise when the version is updated and the package is rebuilt,
the target directory contains two mixed installations.

plugins/go/go.mk

@@ -26,6 +26,7 @@ define $(PKG)_BUILD
         DYLD_INSERT_LIBRARIES= \
         ./make.bash
 
+    rm -rf '$(PREFIX)/$(TARGET)/go'
     mkdir -p '$(PREFIX)/$(TARGET)/go'
     for d in include src bin pkg; do \
         cp -a '$(1)'/$$d '$(PREFIX)/$(TARGET)/go/' ; \