Remove reference to JAEGER_DISABLE_GLOBAL_TRACER

[Sharpie]

This commit removes JAEGER_DISABLE_GLOBAL_TRACER from the jaeger-core
docs. The functionality associated with this setting was removed in
#302.

Signed-off-by: Charlie Sharpsteen source@sharpsteen.net

[codecov]

Codecov Report

Merging #379 into master will decrease coverage by 0.16%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #379      +/-   ##
============================================
- Coverage     84.89%   84.73%   -0.17%     
  Complexity      607      607              
============================================
  Files            94       94              
  Lines          2410     2410              
  Branches        271      271              
============================================
- Hits           2046     2042       -4     
- Misses          272      276       +4     
  Partials         92       92

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/uber/jaeger/senders/ThriftSender.java	75% <0%> (-4.55%)	10% <0%> (ø)
...java/com/uber/jaeger/reporters/RemoteReporter.java	84.7% <0%> (-2.36%)	8% <0%> (ø)

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[yurishkuro]

Thank you for your contribution.