Remove the old output of conv_linker_config

With 3397b6a2aadf8c2eb808fa38f7d6c07d5c82f320 in build/soong,
conv_linker_config fails when the output exists.

For system/vendor partitions, linker.config.pb files are built by
a custom rule, which is missing the removal of the old output. It would
fail with incremental build.

We could fix this by adding --force to overwrite. But I just added `rm`
following the convention in Makefile.

Bug: n/a
Test: m && (touch input of /vendor/etc/linker.config.pb)
      m (again)
Change-Id: Iaeaf861e90b9535394d94c6b16229a4f566789d5