meson: drop explicit python interpreter

meson: drop explicit python interpreter

meson wraps python scripts already on win32, so we end up with these
failing commands:

[185/868] Generating src/rpc/virnetprotocol.h with a custom command
FAILED: src/rpc/virnetprotocol.h
"sh" "libvirt/scripts/meson-python.sh" "F:/msys64/ucrt64/bin/python3.EXE" "F:/msys64/ucrt64/bin/python.exe" "libvirt/scripts/rpcgen/main.py" "--mode=header" "../src/rpc/virnetprotocol.x" "src/rpc/virnetprotocol.h"
SyntaxError: Non-UTF-8 code starting with '\x90' in file F:/msys64/ucrt64/bin/python.exe on line 1, but no encoding declared; see https://peps.python.org/pep-0263/ for details

The issue was introduced in a62486b95feed2cf17ce4adbe794a1ecff9ef22a commit.
These changes are similar as e06beacec2f8e57bbc5cd8f6eb9d44a1f291966d commit.

Signed-off-by: Biswapriyo Nath <nathbappai@gmail.com>