testing/beancount-language-server: remove cargo update -p libc from prepare()

Upstream upgraded to libc 0.2.155 in 1c7fe13d64
so remove this `cargo update` call to prevent accidental downgrades in the future
no pkgrel bump is needed because the aport will be built with libc 0.2.155 regardless of whether `cargo update` is present

testing/beancount-language-server/APKBUILD

@@ -14,9 +14,6 @@ options="!check net" # no test suite, fetch dependencies
 prepare() {
 	default_prepare
 
-	# based on loongarch64 needs
-	cargo update -p libc --precise 0.2.155
-
 	cargo fetch --target="$CTARGET" --locked
 }