Auto merge of #80005 - ssomers:btree_cleanup_3, r=Mark-Simulacrum
Auto merge of #80005 - ssomers:btree_cleanup_3, r=Mark-Simulacrum

BTreeMap: declare clear_parent_link directly on the root it needs

r? `@Mark-Simulacrum`

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust