Remove reset_nice feature

[m-blaha]

I do not understand why we would want such a feature. In default setting the nice value is 0 and this code does nothing. When user runs dnf with "nice" to adjust niceness, he probably has a reason to do so and dnf should not tamper with value set intentionally by user.

[lgtm-com[bot]]

This pull request fixes 2 alerts when merging eb44745b64ce091cbd75a3165bab8eb72094f8da into 034b6538bca70aaf77fdc5ae569fe64e9fa239be - view on LGTM.com

fixed alerts:

• 2 for Except block handles 'BaseException'

[j-mracek]

It looks like that reset_nice was used as a work arround in https://github.com/rpm-software-management/yum/commit/5d7b311e71b0925f4a07bf4ae3c83ccf517034a7

[j-mracek]

I believe that dnf automatic with nice=19 will have a terrible performance. Please see BZ 552279, 742363.