riscv: pgtable: Cleanup useless VA_USER_XXX definitions

riscv: pgtable: Cleanup useless VA_USER_XXX definitions

These marcos are not used after commit b5b4287accd7 ("riscv: mm: Use
hint address in mmap if available"). Cleanup VA_USER_XXX definitions
in asm/pgtable.h.

Fixes: b5b4287accd7 ("riscv: mm: Use hint address in mmap if available")
Signed-off-by: Guo Ren (Alibaba DAMO Academy) <guoren@kernel.org>
Reviewed-by: Jinjie Ruan <ruanjinjie@huawei.com>
Link: https://patch.msgid.link/20251201005850.702569-1-guoren@kernel.org
Signed-off-by: Paul Walmsley <pjw@kernel.org>