remoteproc: core: Remove unused export of rproc_va_to_pa

remoteproc: core: Remove unused export of rproc_va_to_pa

Commit 086d08725d34 ("remoteproc: create vdev subdevice with specific dma
memory pool") added an export for rproc_va_to_pa. However, since its
introduction, this symbol has not been used by any loadable modules. It
remains only referenced within remoteproc_virtio.c, which is always built
together with remoteproc_core.c.

As such, exporting rproc_va_to_pa is unnecessary, so remove the export.

No functional changes.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/20251016-rproc-cleanup-v3-v3-4-774083716e8a@nxp.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>