fix(subscriptions): allow removing subscriptions inside them

[LemonAppleMo]

I'm sorry, my English is not very good. For a brief expression, I only used $subscribe once at that time, which will lead to exceptions in the subsequent $subscribe. I try to fix it in this way.

[netlify]

✔️ Deploy Preview for pinia-official canceled.

🔨 Explore the source changes: 7906ddf

🔍 Inspect the deploy log: https://app.netlify.com/sites/pinia-official/deploys/61f11aae542d590007e12482

[posva]

This doesn't seem to be necessary. Do you have a boiled down repro?

[codecov-commenter]

Codecov Report

Merging #990 (7906ddf) into v2 (85daefb) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##               v2     #990   +/-   ##
=======================================
  Coverage   99.73%   99.73%           
=======================================
  Files           9        9           
  Lines         382      382           
  Branches       97       97           
=======================================
  Hits          381      381           
  Partials        1        1           

Impacted Files	Coverage Δ
packages/pinia/src/subscriptions.ts	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 85daefb...7906ddf. Read the comment docs.

[LemonAppleMo]

Example: https://github.com/LemonAppleMo/pinia-subscribe

When I subscribe to the store in two components at the same time, clearing one affects the other.