fix(watch): remove instance unmounted short circuit in getter of watchEffect

[yangmingshan]

Ref: #7330 (comment)

This is solution 1, relatively high risk.
Solution 2: #9949

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	89.4 kB (-82 B)	34 kB (-19 B)	30.7 kB (-39 B)
vue.global.prod.js	146 kB (-82 B)	53.3 kB (-51 B)	47.6 kB (+23 B)

Usages

Name	Size	Gzip	Brotli
createApp	49.8 kB (-74 B)	19.5 kB (-35 B)	17.8 kB (-20 B)
createSSRApp	53.1 kB (-74 B)	20.8 kB (-40 B)	19 kB (-30 B)
defineCustomElement	52 kB (-74 B)	20.2 kB (-34 B)	18.5 kB (-37 B)
overall	63.2 kB (-74 B)	24.4 kB (-34 B)	22.2 kB (-50 B)

[yangmingshan]

/ecosystem-ci run

[pikax]

/ecosystem-ci run

[vue-bot]

📝 Ran ecosystem CI: Open

suite	result	latest scheduled
language-tools	❌ failure	❌ failure
nuxt	✅ success	❌ failure
pinia	✅ success	✅ success
quasar	✅ success	✅ success
router	✅ success	✅ success
test-utils	✅ success	✅ success
vant	✅ success	✅ success
vite-plugin-vue	✅ success	✅ success
vitepress	✅ success	✅ success
vue-i18n	✅ success	✅ success
vue-macros	❌ failure	❌ failure
vuetify	❌ failure	❌ failure
vueuse	❌ failure	❌ failure
vue-simple-compiler	✅ success	✅ success

[yyx990803]

I took a deeper look and this is already guarded by this line here https://github.com/vuejs/core/blob/main/packages/runtime-core/src/apiWatch.ts#L333 so it should be safe to remove.