doc: remove mention of "proper signing key"

This key is no-longer in use:
https://lists.linuxfoundation.org/pipermail/bitcoin-core-dev/2023-February/000115.html

Author: fanquake

contrib/verifybinaries/README.md

@@ -1,16 +1,5 @@
 ### Verify Binaries
 
-#### Preparation:
-
-Make sure you obtain the proper release signing key and verify the fingerprint with several independent sources.
-
-```sh
-$ gpg --fingerprint "Bitcoin Core binary release signing key"
-pub   4096R/36C2E964 2015-06-24 [expires: YYYY-MM-DD]
-      Key fingerprint = 01EA 5486 DE18 A882 D4C2  6845 90C8 019E 36C2 E964
-uid                  Wladimir J. van der Laan (Bitcoin Core binary release signing key) <[email protected]>
-```
-
 #### Usage:
 
 This script attempts to download the signature file `SHA256SUMS.asc` from https://bitcoin.org.