Remove lengthEnvVariable for Column as it never works as expected

[kezhenxu94]

• If this pull request closes/resolves/fixes an existing issue, replace the issue number. Closes #.
• Update the CHANGES log.

This was introduced in #5238 and it never works as expected so I just remove this and won't consider it as breaking change.

In #5238 we meant to add lengthEnvVariable as an mechanism to override the column length, but the env var it uses is SW_SLOW_DB_THRESHOLD, which is the duration threshold instead of length threshold, what's more, the content of SW_SLOW_DB_THRESHOLD is something like default:200,mongodb:100 but StorageModels always tries to parse the content of SW_SLOW_DB_THRESHOLD as an integer, which causes the following errors

2022-11-12 04:02:25,306 org.apache.skywalking.oap.server.core.storage.model.StorageModels 164 [main] ERROR [] - [9.3.0-SNAPSHOT-7c8867b] Model [top_n_cache_read_command] Column [statement], illegal value default:0,mongodb:100 of column length from system env [SW_SLOW_DB_THRESHOLD]
2022-11-12 04:02:25,311 org.apache.skywalking.oap.server.core.storage.model.StorageModels 164 [main] ERROR [] - [9.3.0-SNAPSHOT-7c8867b] Model [top_n_cache_write_command] Column [statement], illegal value default:0,mongodb:100 of column length from system env [SW_SLOW_DB_THRESHOLD]
2022-11-12 04:02:25,312 org.apache.skywalking.oap.server.core.storage.model.StorageModels 164 [main] ERROR [] - [9.3.0-SNAPSHOT-7c8867b] Model [top_n_database_statement] Column [statement], illegal value default:0,mongodb:100 of column length from system env [SW_SLOW_DB_THRESHOLD]