Revert "build: Force version.h generation"

This reverts commit d914ee0dbc54e39c698e5a3c887987db98b8bdb7 because it breaks
the build:

  I'm trying to compile the latest source code from git HEAD and I'm heaving
  the following error:

  make -r -C build/release/ all
  make[1]: Entering directory `/home/eduardo/workspace/osv/build/release'
  make[1]: *** No rule to make target
  `/home/eduardo/workspace/osv/build/release/gen/include/osv/version.h',
  needed by `/home/eduardo/workspace/osv/build.mk'.  Stop.
  make[1]: *** Waiting for unfinished jobs....
    GEN gen/include/bits/alltypes.h
  make[1]: Leaving directory `/home/eduardo/workspace/osv/build/release'
  make: *** [all] Error 2

Reported-by: Eduardo Piva <efpiva@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>