Remove Unretained call from key import

[spylogsster]

created from comment brave/brave-core#8777 (comment)

[stephendonner]

Verified PASSED using the testplan from brave/brave-core#8858 with build

Brave	1.26.43 Chromium: 91.0.4472.77 (Official Build) beta (x86_64)
Revision	1cecd5c8a856bc2a5adda436e7b84d8d21b339b6-refs/branch-heads/4472@{#1246}
OS	macOS Version 11.4 (Build 20F71)

Steps:

1. new profile, launched Brave
2. opened brave://settings/ipfs
3. loaded ipns://brantly.eth
4. clicked on Use a local node
5. downloaded http://mirror.sfo12.us.leaseweb.net/speedtest/10000mb.bin
6. while downloading, closed the Settings tab
7. waited for shared IPFS link to get copied to clipboard: https://dweb.link/ipfs/QmdBjHTfAp1v4DRBeraqSpXqDS4VvHa4RahTTUnZW3Kwsj?filename=10000mb.bin

Confirmed no crash while doing so; was able to open and save the 9.3 GB file from the above.

example	example	example