gen_syscalls.py: Remove semicolons, unused variables, and a '!= None'

Fix new pylint warnings:

    scripts/gen_syscalls.py:195:0: W0301: Unnecessary semicolon
    (unnecessary-semicolon)

    scripts/gen_syscalls.py:249:0: W0301: Unnecessary semicolon
    (unnecessary-semicolon)

    scripts/gen_syscalls.py:251:0: W0301: Unnecessary semicolon
    (unnecessary-semicolon)

    scripts/gen_syscalls.py:253:0: W0301: Unnecessary semicolon
    (unnecessary-semicolon)

    scripts/gen_syscalls.py:264:0: W0301: Unnecessary semicolon
    (unnecessary-semicolon)

    scripts/gen_syscalls.py:265:0: W0301: Unnecessary semicolon
    (unnecessary-semicolon)

    scripts/gen_syscalls.py:152:8: W0612: Unused variable 'i'
    (unused-variable)

    scripts/gen_syscalls.py:231:18: W0612: Unused variable 'argname'
    (unused-variable)

    scripts/gen_syscalls.py:344:7: C0121: Comparison to None should be
    'expr is not None' (singleton-comparison)

None is a global object, so 'is' (which is like a pointer comparison) is
cleaner, and more idiomatic Python.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>