configs: use default prefix for Windows compilation

The update to Python 3.13 causes meson configuration to fail, see e.g.:

   https://gitlab.com/qemu-project/qemu/-/jobs/12672816538#L397

   meson.build:1:0: ERROR: prefix value '/qemu' must be an absolute path

This is https://github.com/mesonbuild/meson/issues/14303.  Remove the
prefix='/qemu' line in configs/meson/windows.txt, since commit d17f305a26
("configure: use a platform-neutral prefix", 2020-09-30) says that the
NSIS installer doesn't care.

Cc: qemu-stable@nongnu.org
Cc: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20260112160736.1028280-1-pbonzini@redhat.com>
(cherry picked from commit 894c8bd56f)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

configs/meson/windows.txt

@@ -3,7 +3,6 @@
 
 [built-in options]
 bindir = ''
-prefix = '/qemu'
 
 [project options]
 qemu_suffix = ''