Simplify "bool x = y ? true : false". Remove unused function and trailing semicolon.

Summary:

• [tests] Remove unused function InsecureRandBytes(size_t len)
• Simplify "bool x = y ? true : false" to "bool x = y"

This is a backport of Core PR10553

Test Plan:

make check

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3137