Remove unused (and currently broken) `samplecode` target.

Running 'm samplecode' fails with
"build/make/core/main.mk:1834: error: overriding commands for target
`out/target/common/samples/libsimplejni.so', previously defined at
build/make/core/main.mk:1834"
This proves that noone has been using 'samplecode' target at least since
the build started generating both 32- and 64-bit shared libraries by default,
because development/samples/SimpleJNI/jni/Android.mk defines libsimplejni.so
as 'sample', and the deleted code in core/main.mk causes duplicate rule in
such case.

Test: treehugger
Change-Id: I38a4c478fcb0e60793129fe2513a8cdc6d2c79fb