Remove redundant prefix from Python 3 strings

Remove redundant prefix from Python 3 strings

Address the following warnings reported by PyLint 2.10.2:

    ************* Module conf
    doc/arm/conf.py:90:10: W1406: The u prefix for strings is no longer necessary in Python >=3.0 (redundant-u-string-prefix)
    doc/arm/conf.py:92:12: W1406: The u prefix for strings is no longer necessary in Python >=3.0 (redundant-u-string-prefix)
    doc/arm/conf.py:93:9: W1406: The u prefix for strings is no longer necessary in Python >=3.0 (redundant-u-string-prefix)
    doc/arm/conf.py:139:31: W1406: The u prefix for strings is no longer necessary in Python >=3.0 (redundant-u-string-prefix)
    doc/man/conf.py:33:10: W1406: The u prefix for strings is no longer necessary in Python >=3.0 (redundant-u-string-prefix)
    doc/man/conf.py:38:12: W1406: The u prefix for strings is no longer necessary in Python >=3.0 (redundant-u-string-prefix)
    doc/man/conf.py:39:9: W1406: The u prefix for strings is no longer necessary in Python >=3.0 (redundant-u-string-prefix)

(cherry picked from commit e5944bc610fa45902e89c4173d0b7b124876c82b)