remove useless -openmp build option for MRtrix v3.x

[FokkeDijkstra]

Removed the openmp build flag as MRtrix itself does not contain any OpenMP code. Using the flag led to issues with unresolved omp symbols however.

See https://community.mrtrix.org/t/error-population-template-undefined-symbol-omp-get-max-threads/6128/7 for a description of the issue.

I assume the issue is caused by a missing -fopenmp flag on some linker steps in the MRtrix build process. And since no direct OpenMP calls are made in the code it is upon the compiler to decide to actually link OpenMP, or not.
But since MRtrix does not support OpenMP directly it may be better to not use the flag at all.

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS MRtrix-3.0-rc-20191217-foss-2019b-Python-2.7.16.eb
• SUCCESS MRtrix-3.0-rc-20191217-foss-2019b-Python-3.7.4.eb
• SUCCESS MRtrix-3.0.0-foss-2020a-Python-3.8.2.eb
• SUCCESS MRtrix-3.0.3-foss-2021a.eb

Build succeeded for 4 out of 4 (4 easyconfigs in total)
node3136.skitty.os - Linux RHEL 8.6, x86_64, Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz (skylake_avx512), Python 3.6.8
See https://gist.github.com/e1ed41b0932844f0575f1763b8e0f76a for a full test report.

[boegel]

lgtm