fix: avoid unnecessary napi6 feature activation

[wan9chi]

Motivation

vt_client_napi does not use N-API 6-only APIs. Keeping napi6 enabled became observable to downstream workspaces after the addon moved from a build dependency to a normal artifact dependency: Cargo now unifies it into their normal N-API feature graph.

In Vite+, that feature unification exposes ValueType::BigInt in Rolldown's N-API build and breaks its exhaustive match. Remove the unnecessary feature so downstream consumers keep their intended N-API surface.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.51%  [ -3.68% ..  +5.79%]  overhead   +49.57%
dynamic/access             change  -0.33%  [ -3.51% ..  +4.73%]  overhead    +5.92%
dynamic/access-relative    change  -0.07%  [ -1.07% ..  +1.46%]  overhead   +52.12%
static/launch              change  +0.04%  [ -5.68% ..  +7.15%]  overhead  +144.75%
static/access              change  +0.00%  [ -2.38% ..  +2.78%]  overhead  +742.42%
static/access-relative     change  +0.13%  [ -1.15% ..  +1.79%]  overhead +1176.07%

macos

dynamic/launch             change  -0.61%  [ -5.72% ..  +5.61%]  overhead  +244.67%
dynamic/access             change  -2.31%  [-59.45% .. +37.10%]  overhead    +4.76%
dynamic/access-relative    change  +0.90%  [ -4.14% .. +15.77%]  overhead  +238.46%

windows

dynamic/launch             change  +1.21%  [-10.76% .. +12.97%]  overhead   +26.72%
dynamic/access             change  -0.17%  [-14.37% .. +11.25%]  overhead    +2.15%
dynamic/access-relative    change  +0.11%  [-14.62% ..  +4.17%]  overhead    +1.77%