TiDBContext.stmt don't have an eviction mechanism

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

infinitely create prepare stmt
```
create table t(a int);
for i in (10000000):
prepare stmt+str(i) from "select * from t";
```

### 2. What did you expect to see? (Required)

### 3. What did you see instead (Required)
tidb will oom

### 4. What is your TiDB version? (Required)

```
Release Version: v6.2.0-alpha-516-g7cc526a
Edition: Community
Git Commit Hash: 7cc526a9a52256c0e0bd49bb166063fa5b265f2e
Git Branch: release-6.2
UTC Build Time: 2022-08-09 01:03:36
GoVersion: go1.18.2
Race Enabled: false
TiKV Min Version: 6.2.0-alpha
Check Table Before Drop: false
```