[2.7] Drop confusing commented out code in pystrtod.c (GH-6072)

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(cherry picked from commit 9fb8415)

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Fix the following warning:

Python/pystrtod.c: In function 'format_float_short':
Python/pystrtod.c:1007:13: warning: 'strncpy' output truncated before terminating nul copying 3 bytes from a string of the same length [-Wstringop-truncation]
             strncpy(p, "ERR", 3);