[Release] Remove undefined variable check from verify script

[asfimport]

External shell scripts may refer unbound variable:

/tmp/arrow-0.14.0.yum2X/apache-arrow-0.14.0/test-miniconda/etc/profile.d/conda.sh:
line 55: PS1: unbound variable

https://lists.apache.org/thread.html/ebe8551eed2353b248b19084810ff454942b55470b9cf5837aa6cf79@%3Cdev.arrow.apache.org%3E

Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou

Related issues:

• [Release][OSX] Verification script fails to activate conda environment (duplicates)

PRs and other links:

• GitHub Pull Request #4773

_{Note: This issue was originally created as ARROW-5820. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 4773
#4773